You have blackgleam/infernal blow/burning arrow, making 100% of your physical damage convert to fire. This means all your damage is scaled with the weapon elemental damage support gem.

You normally use fire damage to create a huge burn, and can create a bigger burn with avatar of fire. Discharge builds often only scale burn damage, so making useless lightning and cold damage into more fire damage is very good.

At gem level 6 or higher, combined with Avatar of Fire, grants 100% conversion of cold to fire. The most popular use of this is Ice Spear, due to its massive 2nd-form critical chance bonus, to easily apply ignites. All other cold spells will work, but don't provide much utility beyond similar fire spells.

Say you are using infernal blow. 125% damage effectiveness, x% increased Physical Damage on levelling the gem, and 50% physical converted to fire.

After the damage calculations, let's say you deal 1000 physical damage. 50% of that is converted to fire so you deal 500 phys, 500 fire.

If you were to take avatar of fire at this point, another 50% of your physical would be converted to fire, and you would deal only fire damage. so with infernal blow you would deal 1000 fire damage.

100% of your physical damage would be CONVERTED to fire with infernal blow now, so your physical damage still dictates the amount of damage you deal, it simply deals it in fire now.

As for cold damage, you could potentially run Ice spear - cold to fire - ele profile - burning damage - lmp. or something like that, snag avatar of fire, and your ice spear is now doing 122% of the cold damage as fire instead at level 20 cold to fire. Without cold damage, the crits would no longer shatter the targets (stopping the prolif effect), and you could run a wonky fire spear build.

This keystone is not intended to be good with all builds, it is intended to facilitate interesting builds should you have the urge to experiment with them. Hope this helps.

Maybe I'm just not understanding something here, but 50% of 3 different things doesn't make 150% of those 3 things added together, it's still 50%. For example, if you were dealing 10 cold, 10 lightning, and 10 physical damage, your total damage is 30. If you then you got Avatar of Fire, you would be dealing 15 fire damage. Your total damage would be reduced 50%. What is this 150% you are talking about. Yes, your total damage would be reduced by 15, which is 150% of any one of your cold, lightning, or physical damage, but I don't see how that is relevant. It is still only 50% of the 3 added together, and the percentage of damage lost from any one element is still only 50%.

Edit: To put it another way:

You were doing 30 damage.
Now you are doing 15 damage.
15 is 50% of 30.
What exactly did you lose 150% of?

So you are saying that the damage you lose is 150% of the damage originally done by any single element. That is true, but in what way is that relevant? Surely the thing that is relevant is the damage lost compared to the TOTAL damage you were previously dealing?

I feel like this node is missing some sort of damage resistance to go along with doing only fire damage. Maybe gain immunity to fire (almost like Chaos Inoculation) or at least 10 - 15 to all resistances (including chaos and some damage reductions). I like the node, it just feels like it's missing some defense.

AoF is bad, even for IB mara. DPS increase isnt very big (without going into many+fire dmg nodes, and it is easier to get good dps with phys nodes) and you can't leech mana/liffe from fire dmg without additional gem